The electrophysiological principles of the electrical stimulation test in the epidural compartment

Abstract

To the Editor, I commend Margarido et al. on their recent paper in which they evaluated the ability of the epidural stimulation test (EST) to predict local anesthetic consumption. The authors verified that the EST is ‘‘an efficient tool to confirm the epidural catheter placement in the epidural space.’’ Nevertheless, the finding that the EST cannot predict local anesthetic consumption is not surprising since it shows epidural analgesia acting in a compartment phenomenon, as would be expected. As shown in the original article that appeared in the Journal 15 years ago, the fundamental objective of the EST is to confirm catheter tip placement in the epidural space. When the catheter tip is in the epidural space, currents C 1 mA are needed to elicit muscle twitches, whereas motor responses observed at 1 mA or just above 1 mA are usually indicative of placement in the subarachnoid or subdural space or even migration out of the epidural space but still proximal to a nerve root. Regardless, there is no electrophysiological basis to suggest that threshold current itself can predict amount of local anesthetic needed. Volume, concentration, spread of local anesthetic, and epidural opioids all affect analgesia at the corresponding spinal level, but they do not intensify the threshold current provided the current is within the range associated with the epidural compartment (e.g., 1-10 mA). The primary mechanism of the EST relies on stimulation of the nerve root rather than the spinal cord, and the test facilitates predicting the spinal level of the catheter within the epidural compartment. A lower current may be indicative of proximity to the nerve root and may therefore suggest that a lower volume of local anesthetic is needed; however, it is important to account for the fact that effective epidural analgesia typically requires bilateral coverage and that lower volumes may result in unilateral block. As such, although only one side of the nerve root is stimulated with low current, a set volume of local anesthetic would still be required to ensure circumferential spread to cover the opposite side of the nerve root within the compartment. Otherwise, there is the risk of a unilateral epidural block when applying only a small volume of local anesthetic when the threshold current is low. Beyond this clarification, it is important to remind the reader of the significance of the electrophysiological mechanism of the EST and of the importance to evaluate the test accordingly and appropriately.